Rhinovirus illness in young adults was characterized by rhinorrhea, nasal obstruction, sneezing, and pharyngeal discomfort. Length of illness ranged from 1 to 33 days, with a median of 7.42 days; one fourth of the illnesses lasted two weeks. Nasal symptoms, hoarseness, and cough occurred more frequently with confirmed rhinovirus common colds than with illnesses from which rhinoviruses were not cultured. Significant neutralizing antibody responses were measured in 77% of 77 paired serum specimens from patients with rhinovirus illness; only 5% had homologous titers of 8 or more in the acute phase of illness. The findings support other data indicating that serum neutralizing antibody develops following the majority of rhinovirus illnesses and is important in preventing symptomatic rhinovirus infection.